Clearance values of theophylline and its metabolites will be measured in the presence or absence of diltiazem and with high and low doses. The hypotheses tested are: (1) Theophylline clearance is decreased in the presence of diltiazem. (2) The pattern of theophylline metabolite formation and clearance changes in the presence of diltiazem. (3) Diltiazem affects different forms of the cytochrome P450 system. (4) Enzyme systems responsible for theophylline clearance are inhibited by diltiazem in a dose-dependent manner.